This invention relates generally to devices and processes for controlling quality parameters in dewatering plants. More particularly, the present invention relates to devices and processes for controlling the basis weight cross-profile, the formation, the fiber direction and other quality parameters of a pulp sheet after the headbox in paper, tissue and pulp dewatering plants, with at least one suspension feed, at least one distribution device and at least one drainage means for excess suspension from the distribution device, where the distribution device has a distribution chamber and a turbulence-generating chamber made up of a number of pipes adjacent to one another.
A number of facilities are known for controlling the pulp feed to a pulp sheet. DE 199 23 149 (Voith Sulzer Papiertechnik), for example, describes a device where the pressure is measured in the headbox nozzle and the flow rate of the excess pulp suspension drained off is controlled. This backflow is necessary in order to obtain virtually uniform distribution of pressure over the paper sheet width at the distribution device and thus, permit even flow of the pulp suspension onto the wire. In the peripheral zone (edge zones on tender and drive sides), however, turbulence occurs which leads to uneven concentrations and thus, to uneven basis weight, etc.
The aim of the invention is to improve or prevent the uneven quality characteristics discussed above.
The invention is thus characterized by the two peripheral zones of the headbox, comprising distribution chamber and turbulence-generating chamber, each having a discharge means in the sheet running direction for a partial suspension stream. By removing the suspension at the edges in sheet running direction, additional turbulence is prevented and the corresponding uneven zone at the edges is not fed to the pulp sheet, particularly the tissue or paper sheet.
A favorable further development of the invention is characterized by the distribution chamber extending beyond the sheet width on both sides of the headbox by the width of the irregular zones at the sheet edges, where the turbulence-generating chamber can also be widened over the sheet width on both sides of the headbox by the width of the irregular zones at the sheet edges and there can be several pipes in each case in these areas. As a result, those irregularities that are generally found at the edges of the pulp sheet are outside the sheet width that is to be further processed.
An advantageous configuration of the invention is characterized by the pipes at the edges of the turbulence-generating chamber being shortened in comparison to the other pipes, particularly to approximately one quarter of the length of the other pipes. The irregular pulp sheet edges are removed at the earliest possible point in the headbox and thus, are not fed through the headbox and on to the paper machine.
An advantageous further development of the invention is characterized by collecting lines being connected in each case to the outer pipes in the turbulence-generating chamber, particularly to the shortened pipes, where control fittings can be provided in the collecting lines. With these fittings it is possible to set the same pressure drop in the collecting lines at the side as prevails in the rest of the headbox and thus, prevent irregularities in inflow at the transition from the short to the long pipes in the turbulence generator.
A favorable configuration of the device according to the invention is characterized by appropriate discharge means being provided for each layer separately with multi-layer headboxes.
In an operating mode with differential speeds in the individual layers of a multi-layer headbox, different pressure drops occur in each layer. Thus, it is necessary to also be able to set the pressure drops separately in the discharge means at the sides.
The invention also refers to a process for controlling the cross-machine basis weight profile, the formation, the fiber orientation and other quality parameters of a pulp sheet after the headbox in paper, tissue and pulp dewatering machines, with at least one suspension feed, at least one distribution device and at least one discharge means for removing excess suspension from the distribution device.
It is characterized by one partial stream of the suspension being drained off in sheet running direction in both edge zones of the headbox, where the stream being drained off is the one that suffers turbulence due to deflection at the edges of the distribution device.
An advantageous further development of the invention is characterized by the partial streams removed being regulated. This allows the pressure losses at the lateral drainage points to be adapted to the remaining section of the headbox, thus preventing uneven flow conditions at the transition from the short to the long pipes in the turbulence block.
A favorable configuration of the invention is characterized by appropriate partial streams being removed for each layer separately in multi-layer headboxes. In this way, it is possible to set the speeds in the lateral outlets in multi-layer headboxes, which use differential speeds in the individual layers.